This invention relates to the need for alleviating the growing environmental problem of excessive plastic waste that makes up an ever more important volume fraction of what get thrown out in landfills every year. In spite of their environmental awareness, consumers are unwilling to give up the attractive and unique balance of properties and cost that traditional thermoplastics offer. Thus, many of the natural polymers known to offer environmental benefits and degrade rapidly by microorganisms (e.g., cellulose, starch, etc.) have failed to provide a realistic alternative to conventional plastics because they lack their unique set of physical properties (i.e., flexibility, ductility, strength, toughness, etc.), as well as their inherent melt processibility. Therefore, there is a clear need for biodegradable, compostable polymeric thermoplastic materials that would not compromise the convenience of traditional thermoplastics as well as their flexibility, strength and toughness, yet offer alternative solutions to the issue of disposal.
The invention further relates to the need for developing new plastics materials that can be used in applications where biodegradability or compostability among others are part of the primary desirable features of such applications. Such examples include for instance agricultural films, and the convenience that such films offer to farmers when they do not have to be collected after they have served their purpose. Flower pots or seeding templates are other examples where the temporary nature of the substrate translates into convenience for the user. Means of disposal of sanitary garments, such as facial wipes, sanitary napkins, pantiliners, or even diapers, may also be broadened, as such items would advantageously be disposed directly in the sewage, after use, without disrupting current infrastructure (septic tanks or public sewage), hence avoiding handling annoyances and promoting privacy. Current plastics typically used in making such sanitary garments prevent such means of disposal without undesirable material accumulation. New materials to be used in the examples above would ideally need to exhibit many of the physical characteristics of conventional polyolefins; they must be water impermeable, tough, strong, yet soft, flexible, rattle-free, possibly low-cost and must be produced on standard polymer processing equipment in order to be cost-effective.
Another application which illustrates the direct benefit of compostable thermoplastic materials are leaf/lawn bags. Today's sole compostable bag which does not require the composter the additional burden of bag removal and the risk of compost contamination is the paper bag. Yet, it fails to provide the flexibility, the toughness and moisture-resistance of plastic films, and is more voluminous to store. Compostable plastic films used to make leaf/lawn bags would provide bags that could be disposed much like paper bags, yet provide the convenience of plastic bags.
It becomes clear in view of these examples that a combination of biodegradability, melt-processability and end-use performance is of particular interest to the development of a new class of polymers. Melt processability is key in allowing the material to be converted in films, coatings, nonwovens or molded objects by conventional processing methods. These methods include cast film and blown film extrusion of single layer structures, cast or blown film co-extrusion of multi-layer structures. Other suitable film processing methods include extrusion coating of one material on one or both sides of a compostable substrate such as another film, a non-woven fabric or a paper web. Other processing methods include traditional means of making fibers or nonwovens (melt blown, spun bounded, flash spinning), and injection or blow molding of bottles or pots. Polymer properties are essential not only in ensuring optimal product performance (flexibility, strength, ductility, toughness, thermal softening point and moisture resistance) during end-use, but also in the actual product-making stages to ensure continuous operations.
In the past, the biodegradable and physical properties of a variety of PHA's have been studied, and reported. Polyhydroxyalkanoates are semicrystalline, thermoplastic polyester compounds that can either be produced by synthetic methods or by a variety of microorganisms, such as bacteria and algae. Traditionally known bacterial PHA's include Poly(3-hydroxybutyrate), or i-PHB, the high-melting, highly crystalline, brittle, homopolymer of hydroxybutyric acid, and Poly(3-hydroxybutyrate-co-valerate), or i-PHBV, the somewhat lower crystallinity and lower melting copolymer that nonetheless suffers the same drawbacks of high crystallinity and brittleness. Their ability to biodegrade readily in the presence of microorganisms has been demonstrated in numerous instances. They however are known to be fragile polymers which tend to exhibit brittle fracture and/or tear easily under mechanical constraint, They clearly do not qualify as tough, ductile or flexible polymers. Their processability is also quite problematic, since their high melting point requires processing temperatures that contribute to their extensive thermal degradation in the melt. Other known PHA's are the so-called long side-chain PHA's, or PHO's (poly(hydroxyoctanoates)). These, unlike PHB or PHBV, are virtually amorphous owing to the recurring pentyl and higher alkyl side-chains that are regularly spaced along the backbone. When present, their crystalline fraction however has a very low melting point as well as an extremely slow crystallization rate, two major drawbacks that seriously limit their potential as useful thermoplastics for the type of applications mentioned in the field of the invention.
The use of Poly(3-hydroxybutyrate) homopolymer (i-PHB)and Poly(3-hydroxybutyrate-co-valerate) copolymer (PHBV) in blends are described in Dave et al. (Polym. Mater. Sci., 62, 231-35 (1990)) and in Verhoogt et al. (Polymer, 35(24), 5155-69, (1994)). Blending however did not readily resolve the issue of mechanical fragility and lack of flexibility of such high-crystallinity PHA's, while maintaining the biodegradable nature of these materials.
Several patents have made claims with regard to a blending approach for improving the mechanical properties of i-PHB and PHBV, with only mitigated success. Such blend compositions are excluded from this invention.
Tokiwa et al., U.S. Pat. No. 5,124,371, to AIST, Japan (see also JP 03 157450, Jul. 5, 1991), discloses a biodegradable plastic composition made of i-PHB and PCL (polycaprolactone). The optimal use of a third component, such as a copolymerization catalyst, is reported. This composition is excluded from the following patent by Hammond (see U.S. Pat. No. 5,646,217 next), the latter being aimed at expanding the concept of blending to other polymers. Tokiwa's blends of PHB with PCL as well as Hammond's blends fall short in exhibiting the ductility and toughness desired in a large variety of applications, as evidenced by the mechanical properties disclosed in their examples.
Hammond, U.S. Pat. No. 5,646,217, August 1997, to Zeneca (see also WO-A-94 11440, EP 669959 A1 and JP 08503500) discloses polymer compositions which comprise a first polyhydroxyalkanoate component and optionally a second polymer component, the compositions have enhanced properties by using an inorganic oxygen containing compound in the composition. The inorganic oxygen-containing compound may be acting as a transesterification catalyst. It is an oxy compound of a metal from group IIA, IIIA or IVA of the Periodic Table or a metalloid having a valency of at least 3 from a B group of the Periodic Table. The PHA's are said to have chemical repeating units of the following formula:[—O—CmHn—CO—], m=1-13 ; n=2m or 2m−2(m>2);     with specific mention of PHB and PHBV chemical structures.
In the present invention, we have unexpectedly discovered that, for the less crystalline and more ductile randomly altered PHA copolymers of lower crystallinity than i-PHB and i-PHBV, there is no need for the addition of a transesterification catalyst to achieve excellent mechanical compatibility in blends with aliphatic ester polycondensates. Moreover, such blends exhibit truly outstanding mechanical properties, especially toughness and flexibility, that are not only far superior to any disclosed in Hammond's patent, but also that can compete favorably with polyolefins, such as LLPDE (linear low density polyethylene) or i-PP (isotactic polypropylene). For instance, in all examples cited in Hammond's patent, the elongation at break of all blends fails to surpass 20% and reported toughness measurements are generally mediocre. To the contrary, our blends exhibit elongation at break values up to several 100% and toughness values that can actually surpass that of polyolefins. In addition, improvement in crystallization in the blend compositions of the present invention also far surpasses those described in Hammond's patent, and our blends can be easily processed from the melt at a lower temperature without extensive thermal degradation, making them preferred materials for high performance, disposable, biodegradable and/or compostable products.
Hammond, U.S. Pat. No. 5,550,173 to Zeneca, May 1996, (also WO 94/11445, EP 668893A1), discloses a polymer composition comprising a polyhydroxyalkanoate having a molecular weight of at least 50,000 and at least one oligomer of a polymer selected from the group consisting of polyhydroxyalkanoates, polylactide, polycaprolactone and copolymers thereof. Such oligomers have molecular weight 2,000 or less, are non-volatile and have lower Tg's that the PHA's to be modified. Oligomers are said to contribute to increase the flexibility of PHA's by lowering the Young's modulus, i.e. the modulus of elasticity. They also contribute to accelerate the biodegradation process, while being non-volatile additives. Based on the patent's data, there is no significant improvement in toughness associated with the addition of selected oligomers (see elongation at break data or Izod impact data in table 7). In addition, the disclosed oligomer structures do not include those based on ester polycondensates, one of the blend components of the present invention.
Montador et al., U.S. Pat. No. 5,516,825 to Zeneca, May 1996 (also EP655077), disclose biodegradable polyesters derived from hydroxy alkenoic acids which may be plasticized with an esterified hydroxycarboxylic acid which has at least three ester groups, at least some of the hydroxy groups being esterified with a carboxylic acid and at least some of the carboxy groups being esterified with an alcohol and/or phenol.
Along the same idea of plasticization, Hammond et al., U.S. Pat. No. 5,753,782 to Zeneca, May 1998, (also EP 701586A1, WO 94/28061) disclose polyester composition comprising a biodegradable polyester and a plasticising quantity of at least one plasticiser selected from the group: high-boiling esters of polybasic acids; phosphoric acid derivatives; phosphorous acid derivatives; phosphonic acid derivatives; substituted fatty acids; high-boiling glycols, polyglycols, polyoxyalkylenes and glycerol each optionally substituted and optionally terminally esterified; pentaerythritols and derivatives; sulphonic acid derivatives; epoxy derivatives; chlorinated paraffins; polymeric esters; Wolflex-But*; provided that citrates does not include doubly esterified hydroxycarboxylic acids having at least 3 ester groups in its molecule and further provided that glycerols does not include glycerol triacetate and glycerol diacetate. In both patents, improvement in overall mechanical properties were reported (elongation at break, impact data) along with a more significant reduction in stiffness (drop in Young's modulus). Yet, elongation at break data, for instance, remain below 100%, and Izod impact data only increase 2-4 fold. This is well below the over 10 fold toughness improvement that is typically necessary for commercial applications.
Matsushita et al, JP 08-157705 to Mitsubishi Gas & Chem. (June 1996), disclose a biodegradable resin composition comprising an aliphatic polyester prepared from a glycol, an aliphatic dicarboxylic acid or its derivative and poly-3-hydroxybutyrate. It is desirable that the poly-3-hydroxybutyrate has a weight-average molecular weight of 400 k g/mole or above. If it has a molecular weight below that, it reportedly cannot give a satisfactory molding The purpose was to obtain a biodegradable resin composition excellent in moldability, mechanical properties and heat resistance by mixing a specified aliphatic polyester with poly-3-hydroxybutyrate. Blends of i-PHB, the homopolymer of hydroxybutyric acid, with polycondensates of glycol and aliphatic dicarboxylic acid are excluded from the present invention, by restricting the definition of PHA's to copolymers of reduced crystallinity and greater ductility and flexibility.
Similarly, Miura et al, JP 8027362A to Mitsubishi Gas and Chem. (January 1996), disclose a composition comprising desirably 99-50 pts.wt. aliphatic polyester carbonate obtained by condensing an aliphatic dibasic acid, desirably succinic acid, with an aliphatic dihydroxy compound, desirably 1,4-butanediol, and a diaryl carbonate (e.g. diphenyl carbonate) and desirably 1-50 pt wt. poly-beta-hydroxybutyric acid. Again, blends containing the stiffest and most brittle member of the PHA family, i.e. i-PHB and PHBV, are excluded from the present invention.
Dabi et al., EP 606923A2 and EP 882765A2, January '1994 to McNeil-PPC, Inc., disclose two classes of thermoplastic biodegradable compositions that are said to exhibit good mechanical properties and readily degrade in the presence of microorganisms. One aspect of the invention discloses biodegradable compositions based on destructurized starch-polymer alloys that are out of the scope of the present invention. Another aspect of the invention provides blends of a thermoplastic and ester containing polymer, a plasticizer and optionally an inert filler. More specifically, these compositions are described as comprising:                10 to 70 wt % polymers or copolymers comprising one or more repeating units of the general formula:[—O—CHR—CH2—CO—]n  (I)         (˜R=1 to 9 carbon-containing alkyl groups);        5 to 35 wt % ester-containing polymers, of molecular weight greater than 10,000 and selected from the group consisting of:                    Polymers with ester linkages in the backbone, of the following type;[—O—CO—R1—CO—O—R2—]n  (II)            Polymers with pendant ester groups, of the following type:[—CH2—CHX—CH2—CHOCOCH3—]n  (III)and[—CH2—CR4COOR5—]n  (IV)                        0 to about 30 wt % of one or more plasticizers, such as triacetin;        0 to about 50 wt % of an inert filler, such as calcium carbonate or starch;        
Examples that illustrate such compositions include PHBV (commercially available Biopol) blended with either PCL (polycaprolactone) or EVA (ethylene-vinylacetate copolymer). Both polymers are outside the scope of the present invention. The mechanical properties achieved, although better than for pure PHBV, fail to be outstanding and would be unlikely to compete with polyolefins, whether on toughness or flexibility, based upon the available data. Only in very limited cases did the elongation at break of the blends surpass 100%; and in no instance was 300% elongation reached. In fact, in the 70/30 blend of PHBV and PCL without additives, the reported elongation at break of 15% is indicative of brittle fracture (no ductility).
Polybutylene succinate or polybutylene succinate-co-adipate, the most preferred embodiments of the present invention with regard to the type of ester polycondensates to be blended with our PHA copolymers (see the detailed description of the invention further below) is neither cited in the patent nor is it used in examples.
Hence, the authors of the above invention fail to recognize and establish how the novel PHAs of the present invention and which differ from PHB or PHBV in both their chemical structure and mechanical performance, are capable of achieving truly discontinuous outstanding mechanical properties in blends with ester polycondensates such as polybutylene succinate or polybutylene succinate-co-adipate. Performance-wise, the surprising result is that such blends capable of surpassing not only those of similar blends with conventional PHA's like PHB or PHBV, but also those of common ductile polyolefins such as polyethylene or polypropylene, as illustrated in the examples below. In addition, blends of the present invention compete favorably in terms of their ability to undergo rapid biodegradation, and can be easily processed, making them preferred materials for high performance, disposable products.
Tsai et al., World Patent Application No WO 98/29493 to Kimberly-Clark (July 1998) disclose a thermoplastic composition that comprises a unreacted mixture of an aliphatic polyester polymer and a multicarboxylic acid. One example of such a thermoplastic composition is a mixture of poly(lactic acid) and adipic acid. The thermoplastic composition is capable of being extruded into fibers that may be formed into nonwoven structures that may be used in a disposable absorbent product intended for the absorption of fluids such as body fluids. The second claim discloses a composition made of a variety of aliphatic ester polymers, and mixtures thereof, as well as copolymers of such polymers. Bionolle and PHBV are among the polymers listed, their blend being outside the scope of the present invention. Other less crystalline and more flexible PHA's are not cited.
Wu et al., U.S. Pat. No. 5,200,247 June 1992 (also EP 882765) to Clopay Plastics Prod. Co., disclose a biodegradable thermoplastic film comprising a blend of an alkanoyl polymer and poly(vinyl alcohol). The film can be stretched providing opacity and enhancing its biodegradability. The alkanoyl thermoplastic polymer, which is said to make up 90-75 wt % of the blend, selected form the group consisting of:                a) dialkanoyl polymer (at least 10% of recurring dialkanoyl units),        b) oxyalkanoyl of formula O(CH2)xC═O (x=2-7),        and mixtures thereof.The above definition does not include the specific PHA copolymers of the present invention, and in its most preferred embodiment, the oxyalkanoyl polymer is PCL, (i.e. polycaprolactone). There is no specific claim of film performance beyond the fact that the film must be ductile in order to be stretchable.        
Matsumura et al, U.S. Pat. No. 5,464,689 to Unicharm Corp. November 1995 (also EP 0 629 662B1 and JP7003138), disclose a resin composition which comprises 40 to 85% PHBV (8-15% V); 60 to 15% PCL and 5-40 vol. % of inorganic filler (part. size of 0.1 to 10 micron), and porous films produced from the composition by a disclosed stretching process. The authors claim that porous film to be easily my microorganisms. Such biodegradable polyester blends are outside the range of materials and compositions included in the present invention.
Kleinke et al., U.S. Pat. No. 5,231,148, to PCD Polymere Gesellschaft (November 1991), disclose mixtures comprising at least 70% by weight of a polyhydroxyalkanoate and 0.1 to 10% by weight of a compound or a mixture of compounds which contain at least two acid and/or alcohol groups, which are melted or softened and/or dissolved in a melt of said polyhydroxyalkanoate and/or are miscible with the melt at the melting point of said polyhydroxyalkanoate, mixtures of poly-D(−)-3-hydroxybutyric acid with a polyether being excluded. The ester polycondensates of the present invention are generally neither soluble nor miscible with the PHA's copolymers, and there is no clear evidence of chemical reactions taking place.
Yoon et al, J. Poly. Sci., Pol. Phys., 34, pp 2543-2551 (1996) have examined compatibility and biodegradability aspects of blends of i-PHB with an aliphatic terpolyester of adipic acid, ethylene glycol and lactic acid. They determine that such polymers were considered compatible from structural studies, yet did not observe any chemical changes such as transesterification as a result of blending.
Kumagai et al., Polymer Degradation and Stability, 36, p. 241 (1992) disclose blends of poly(3-hydroxybutyrate) with either poly(□-caprolactone), poly(1,4-butylene adipate) or poly(vinyl acetate). In the first two cases, blends are found to be immiscible, whereas miscibility was observed in blends of the third kind. In a parallel study, Kumagai et al., Polymer Degradation and Stability, 37, p. 253 (1992), disclose blends of poly(3-hydroxybutyrate) with poly(b-propiolactone) , poly(ehtylene adipate) or poly(3-hydroxybutyrate-co-valerate) with high HV content. The authors disclose that rates of enzymatic degradation of films formed from the blends are higher than the rate of each polymer component film.
Wnuk et al., World Patent Applications Nos. WO 96/08535 and WO 97/34953, disclose general compositions comprising blends of biodegradable polymers, and exemplify polymer compositions comprising a biodegradable polyhydroxyalkanoate and a second biodegradable polymer selected from the group consisting of aliphatic polyester-based polyurethanes, polylactides, polycaprolactone and mixtures thereof. The aliphatic polyester-based polyurethanes referred to above are low crystallinity, thermoplastic elastomer-like grade that differ from the semicrystalline polyesters of the present invention that contain a majority of aliphatic dialkanoyl recurring units. In particular, such polyurethanes cannot contribute to an increase in crystallization rate similar to that described in one of the examples of the present invention. Also, there is no differentiation made between the low performance of blends made using conventional, highly crystalline, brittle PHA's (such as PHB of PHBV) and the much greater ductility and toughness of blends of the present invention that comprise lower crystallinity PHA's.
Finally, with regard to polyester blends, Hubbs et al., World Patent Application No WO 94/00506 to Eastman Kodak, disclose a variety of blends of PHA's with other polyesters, including aliphatic ester polycondensates. The PHA's disclosed are made solely by chemical synthesis only and are atactic in nature, i.e. with no optical activity, hence exhibiting little or no crystallinity. They differ from the PHA's of the present invention, which are either fully isotactic, i.e. optically pure, when made via biosynthesis, or largely isotactic (97%) when specific catalysts such as alkylzinc alkoxides are used to polymerize b-substituted b-propiolactones (see U.S. Pat. No. 5,648,452, L. A. Schechtman et al., assigned to the Procter and Gamble Co.).
Recently, new poly(3-hydroxyalkanoate) copolymer compositions have been disclosed by Kaneka (U.S. Pat. No. 5,292,860), Showa Denko (EP 440165A2, EP 466050A1), Mitsubishi (U.S. Pat. No. 4,876,331) and Procter & Gamble (U.S. Pat. Nos. 5,498,692; 5,536,564; 5,602,227; 5,685,756). All describe various approaches of tailoring the crystallinity and melting point of PHA's to any desirable lower value than in the high-crystallinity PHV or PHBV by randomly incorporating controlled amounts of “defects” along the backbone that partially impede the crystallization process. Such “defects” are either, or a combination of, branches of different types (3-hydroxyhexanoate and higher) and shorter (3HP, 3-hydroxypropionate) or longer (4HB, 4-hydroxybutyrate) linear aliphatic flexible spacers. The results are copolymer structures that undergo melting in the most useful range of 80° C. to 150° C. and that are less susceptible to thermally degrade during processing. In addition, the biodegradation rate of these new copolymers is typically improved as a result of their lower crystallinity and the greater susceptibility to microorganisms. Yet, whereas the mechanical properties of such copolymers are improved over that of PHB or PHBV, their toughness remains inferior to that of polyolefins as for instance after prolonged physical aging. Aging is responsible for the stiffening of these copolymers, which further affect their ductility, i.e. their ability to undergo large-scale plastic deformation without undergoing failure. It mimics the aging effect reported for PHB and PHBV by G. J. M. deKoninck et al, although to a lesser extent. In World Patent Application WO 94/17121, the latter disclose a thermal annealing treatment capable of partially reversing the aging effect which nevertheless falls short of bringing in sufficient ductility in these high-crystallinity polymers. Finally, the rate of crystallization of the new, more suitable, copolymers is characteristically slow and remains a challenge for them to be processed by conventional converting methods.
Despite all these advances in designing more useful PHA copolymers and the like, there still remains a challenge to find a class of materials that exhibits the outstanding polyolefin-like properties (e.g., flexibility, ductility, toughness, water-impermeability) that have come to be expected from thermoplastics, a high rate of biodegradation which opens up alternative approaches to disposal beyond landfill, and processing characteristics that allow them to be easily handled on conventional converting equipment without major transformation. The present invention provides novel compositions which have been found to offer a useful balance of mechanical properties, high biodegradation rate and ease of processability.